1. Field of the Invention
The present invention relates to an object verification method for use in radar systems for motor vehicles, in which the distances and relative velocities of located objects are determined on the basis of received radar echoes.
2. Description of Related Art
In driver assistance systems for motor vehicles, radar systems are used for sensing the surrounding field of traffic, in particular for finding the position of vehicles traveling up ahead and potential obstacles in the near field of one's own vehicle. A typical application example is an ACC system (adaptive cruise control) for automatically controlling the distance to a vehicle in front of one's own vehicle. Moreover, driver assistance systems are being developed, whose primary purpose is to enhance traffic safety. These include automatic collision warning systems or collision avoidance systems, which automatically trigger an emergency braking in response to an imminent collision danger.
When both the distances and the relative velocities of the located objects are to be determined using a radar system, then at least two measured quantities must be derived for each object from the received radar echoes in order to calculate the distance and the relative velocity. When the positions of a plurality of objects are to be found simultaneously, then the problem arises that the measured quantities must be correctly assigned to the respective objects. Incorrect assignments lead to the detection of ghost objects to which no real object corresponds. When working with safety-critical systems, such as collision avoidance systems, such ghost objects must be eliminated in a fail-proof manner, since, otherwise, they would lead to spurious activations, which, in turn, can pose a safety risk.
Methods are known for verifying real objects and for eliminating ghost objects. An example of a radar system widely used in driver assistance systems is a so-called FMCW radar (frequency modulated continuous wave). In this system, the frequency of the transmitted radar signal is modulated in a ramped form, and the received signal is mixed with the signal transmitted at the instant of reception, so that an intermediate frequency signal is obtained, whose frequency corresponds to the difference in the frequency between the transmitted and the received signal. This frequency differential is dependent, on the one hand, on the signal propagation time and, thus, on the distance of the located object and, on the other hand, on the Doppler shift and, thus, on the relative velocity of the object. To uniquely determine the distance and the relative velocity, two measurements are needed, in which different ramp gradients are used. For example, when an operation is performed for a rising and a falling ramp, using the amount following an equal gradient, then the distance-dependent components cancel each other out in the sum of the intermediate frequencies, making it possible for the relative velocity to be determined. If, on the other hand, a subtraction operation is performed to compute the difference between the intermediate frequency signals, then those components which are dependent on the relative velocity are eliminated, and the distance can be calculated. However, if two different objects are located simultaneously, then two intermediate frequencies are obtained per rising and per falling ramp, and two possible pairings are derived for the addition and subtraction operations, so a unique result is not obtained. The “spurious” pairing then corresponds to a ghost object.
This ambiguity can be remedied by modulating the transmitted frequency using a third ramp whose gradient is shifted relative to those of the two other ramps. The intermediate frequency obtained for one single ramp defines a correlation between the distance and the relative velocity. An object is identified as being real when the correlations obtained for the three different ramps are consistent with one another, i.e., form a uniquely solvable system of equations.
However, as the number of simultaneously detected objects increases, there is an increased statistical probability of a system of equations obtained for a ghost object also having a unique solution—within the limits of accuracy. In such a case, it is not possible to eliminate the ghost object using the previously known method.
There are a number of other causes for the occurrence of ghost objects. One common cause is, for example, the so-called “clutter” of fixed target objects which are spaced at intervals along the side of the road, such as guardrail delineators and the like. Such objects can produce superimposed effects, resulting in quasi-randomly produced detections in the individual FMCW ramps. Instance can arise under certain circumstances in which the detections are combined with one another or with the detections of real objects, to form ghost objects.